Birdseye View of the Ice-Age Floods
Total Page：16
File Type：pdf, Size：1020Kb
AftermathAftermath ofof thethe IceIce--AgeAge Floods:Floods: AA Bird’sBird’s EyeEye ViewView Bruce Bjornstad Pacific Northwest National Laboratory Most Ice-Age floods came from repeated outbursts of glacial Lake Missoula. Downstream, the floods poured out over southeastern Washington, scouring out an interconnected network of channelways (i.e., the Channeled Scabland) over a hundred miles wide. Infrared Satellite Image of Washington State The Channeled Scabland is recognized from outer space where the floods completely removed the loose topsoil, eroding down into the underlying dark basalt bedrock. ChanneledChanneled ScablandScabland Landsat Image Land-Use Map of the Central Columbia Plateau N GC GC T-CC C-P T-CC C-P N Floodwaters descended across the Channeled Scabland via three interconnected routes: 1) the Cheney-Palouse (C-P), and 2) Telford-Crab Cr. (T-CC) scabland tracts, and 3) Grand Coulee (GC). During the largest Ice-Age floods all three routes were simultaneously filled with water from periodic outbursts from glacial Lake Missoula. CloserCloser ViewView ofof SouthSouth--CentralCentral ChanneledChanneled ScablandScabland North Be ezley HIlls . C GC a R P Q mbi FC uincy Colu Fr LC ench Bas man in LC ML CC Hills Sadd le Oth ello M Ba DC o sin unt P ains as Above B co as O in flood level EL SR C R Vertical Exaggeration = 4X Floodwaters naturally followed the path of least resistance. This included areas of lower elevation or more easily eroded sediment and rock. Higher elevations escaped the floods’ wrath. GC = Grand Coulee, LC = Lind Coulee, DC = Drumheller Channels, PC = Potholes Coulee, FC = Frenchman Coulee, LCCC = Lower Crab Creek Coulee, EL = Eagle Lakes, SR = Scooteney Reservoir, RC = Ringold Coulee, ML = Moses Lake, O = Othello Telford-Crab Creek Scabland Tract Cheney-Palouse Scabland Tract Over much of the Channeled Scabland the floods completely stripped away the topsoil, exposing and eroding into the underlying, Lower Grand Coulee dark, basalt bedrock. Scabland Palouse Hills Between scabland flood channels the gently rolling Palouse hills, composed of windlblown, fine sand and silt, were preserved in areas that lay above flood level. ScablandScabland FeaturesFeatures e v Mesa o o r G Mesa Butte Mesa Mesa Characteristic of the Channeled Scabland are potholes (arrows), also referred to as kolks, created by violent vortices during flooding. The swirling action created hydraulic lift, which was able to pluck out and transport huge blocks of fractured basalt. Other common scabland features include mesas, buttes, spill-over channels, and grooves. ChannelsChannels DrumhellerDrumheller Southeast Longitudinal ScablandScabland FeaturesFeatures atat Grooves Mesas, Buttes, Basins, and Channels Potholes (Kolks) StreamlinedStreamlined PalousePalouse HillsHills Streamlined Palouse Hills Scabland Scabland Streamlined Palouse hills mark areas where the floods failed to completely remove all the topsoil within flood channels. These “islands” of loose Palouse soil were eroded and molded into their present shape by the deep, rapidly moving Ice-Age floodwaters. Northeast Cheney-Palouse Scabland Tract Palouse R. e cours de rmer vi a Fo e R. Di Palouse shtucn alous Wa of P Falls ulee e Co ous Snake al R. P r Hwy. 261 me For AlteredAltered Unscoured Streamlined Palouse Soil d LandLand-- Floo (above flood red Palouse scou level) ks in Hills crac lt scapescape basa HU Ran HU Ranch ch Coulee Cataract Vertical Exaggeration = 4X Prior to Ice-Age flooding the Palouse Divide was higher and covered with a thick layer of Palouse soil. Also during this time the Palouse R. ran westward down Washtucna Coulee. With Ice-Age flooding, floodwaters smashed into the Palouse Divide. Because Washtucna Coulee could not handle all the water coming down the Cheney-Palouse tract, floodwaters had nowhere else to go but up and over the Palouse Divide. The floods quickly eroded down into the underlying bedrock where they encountered a number of pre-existing deep cracks (red lines). The floods preferentially eroded the weaker basalt along these cracks. Today the Palouse River and HU Ranch Coulee are entrenched along these subparallel cracks. North Diversion of the Palouse R. WC Cheney- For P mer alous of cou e N Palo rse Flood-scoured use Scabl R. WC and cracks in basalt Tract R P P a l o u s PD e Former course WC R . of Palouse R. DC SR PR C Pasco South P R C WA OR The area shown in the photos at left is represented by the rectangle above. Prior to flooding, the Palouse R. flowed southwestward down Washtucna Coulee. The Palouse R. took an abrupt right turn southward after floodwaters eventually cut a lower channel across the Palouse Divide, creating a shortcut to the Snake R. PR = Palouse R., PRC = Palouse River Canyon, SR = Snake R., WC = Washtucna Coulee, DC = Devils Canyon, PD = Palouse Divide Washtucna Coulee Above Devil’sDevil’s flood level DC CanyonCanyon Columbia Wa shtu Plateau Trail cna C oulee Kahlotus DC Entrance to RR Tunnel along Columbia Plateau Trail L. Monumental Dam Snake R. Vertical Exaggeration = 4X As with the Palouse R. Canyon (PRC) to the east, floodwaters moving down Washtucna Coulee (WC) overtopped another divide between WC and the Snake R, carving a new channel, called Devil’s Canyon (DC). The straightness of DC suggests it too developed in weaker rock along a fracture line in the basalt. Unlike the PRC, DC is dry, since the flow of water in WC and DC was cut off after the last Ice-Age floods. GiantGiant CurrentCurrent RipplesRipples H.P. Schwert photo two-lane road Channeled Scabland Snake River The huge scale of giant current ripples, up to several meters high and tens of meters apart, attests to the magnitude of the Ice-Age floods. North West Bar Giant Current ar B nt ce es Cr Ripples at h nc Be . West Bar a R ck bi bco lum Ba Co Vertical Exaggeration = 4X North South SteamboatSteamboat RockRock Northeast Above flood level r e p p GCR’s U d n ra G Banks e Lake le u o C Several sets of giant current ripples (GCR’s) are perched atop Steamboat Rock, in the upper Grand Coulee. These ripples lie 750 ft above Banks Lake, a direct indication of the floods depth at this location. Like the prow of a giant ship, the steep vertical face of Steamboat Rock points in the direction of the floods, a most hydro-dynamically stable form for this monolith. CataractCataract CanyonsCanyons Northeast North Visitor’s Center Dry Falls Frenchman Coulee Northeast Northeast Vertical Exaggeration = 4X Palouse Falls Potholes Coulee Cataract canyons are another flood feature characteristic of Ice-Age flooding. Shown here are four examples of cataract canyons within the Channeled Scabland. Similar features have been observed on Mars, leading scientists to believe huge floods have occurred on Mars as well. PotholesPotholes Coulee/QuincyCoulee/Quincy LakesLakes Northeast Q uin Above flood cy La level kes P B ot ab ho co le ck s C ou lee C olu mb B ia en Riv c er h Vertical Exaggeration = 4X Potholes Coulee is a pair of cataract canyons formed by floodwaters moving west out of the Quincy Basin. Floodwaters also moved down the Columbia River valley from the north, but probably at a lower elevation. CataractCataract ComplexComplex atat PotholesPotholes CouleeCoulee Above Flood Level Cataract P Plunge pool Flood bar P P P P P During flooding cataracts were like giant waterfalls; the floods eroded down to more-resistant layers of basalt. The force of falling water created huge plunge pools that undercut cataracts at their base. This allowed the vertical walls of cataracts to maintain themselves as they migrated upstream. Huge bars of gravel and sand, hundreds of feet deep and up to a mile long, formed downstream of the cataract complex. Northwest Above flood level Cataract cliffs Columbia R. Coulee s Pothole Lakes Quincy Quincy Lakes is in the foreground with Potholes Coulee and Colum and several hundred feet below. In stair cataract cliffs, first into Potholes Coulee, and then into the C -step fashion, floodwaters tumbledbia River over beyond olumbia River. BottleneckBottleneck atat WallulaWallula GapGap Wallula Gap All floodwaters were forced to pass through a single, narrow constriction at Wallula Gap. Elevation Map of Southeastern Washington N ChanneledChanneled WA ID ScablandScabland Lake Lewis Pasco WA Lake OR Condon Wallula Gap During larger floods the volume of water entering the Pasco Basin was greater than could pass through the narrow opening at Wallula Gap, so the water backed up to an elevation of 1200 ft, forming temporary Lake Lewis. Downstream, Lake Condon rose to a 1000-ft elevation behind another hydraulic constriction in the Columbia. R. Gorge. Lakes Lewis and Condon only lasted a few days, or as much time required for all the water to drain out to the Pacific Ocean. WallulaWallula GapGap East Above flood level 1200 ft 300 ft Two Sisters South Columbia R. During the largest floods, water levels in Wallula Gap were up to 900 ft higher than today. Below the high water mark (1200 ft) slopes were stripped clean of soil and scabland-like features, such as mesas, buttes, spill-over channels, and pinnacles (e.g., Two Sisters) are the predominant landforms. Chaotic Network of Spill-Over Channels at Wallula Gap Two Sisters Above flood level Water levels rose to ~1200 ft during the largest floods, the height to where we see flood-scoured spill-over channels. Above 1200 ft lie only smooth, soil-covered slopes, unaffected by the raging torrent below. All floodwaters were forced to pass through this single opening, which at its narrowest, was <3 mi. wide. Kennewick Northwest C olum bi a R ive r Yellepit Pond Northwest end of Wallula Gap as it appears today.